Light emitting semiconductor device and a method for making the same

ABSTRACT

In a method for making a light emitting device having hemispherical dome type geometry, a p-conductivity type and then an n-conductivity type layer are successively grown epitaxially on a substrate made of a mixed compound semiconductor crystal having a band gap wider than the two above-mentioned layers. A surface portion of these epitaxially grown layers, which is not covered by a mask deposited on the n-conductivity type layer at a position where a p-n junction is to be formed, is doped with p-conductivity type impurities so that a small n-conductivity type region is surrounded by a region converted into p-conductivity type. The other side of the crystal is formed into a hemispherical shape so that the n-conductivity type region is located at the central portion of the hemisphere.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light emitting semiconductor device,more particularly, to a high output light emitting semiconductor devicemade of a III-V mixed compound semiconductor crystal having a p-njunction, and to a method for making the same.

2. Description of the Prior Art

Various types of light emitting semiconductor devices are already known.They are often made of a III-V mixed compound semiconductor crystal,such as GaAs₁ _(-x) P_(x) and Ga₁ _(-x) Al_(x) As, having a dome orhemispherical shape so that light generated at a p-n junction, which isformed mostly at the center of the planar surface of the hemisphere, canbe emitted outside the crystal without total reflection at the surfaceof the crystal. In one of these hemispherical dome type geometrydevices, the major part of the crystal is of n-conductivity type, and ap-conductivity type region is formed by diffusion at the central portionof the hemispherical crystal. One electrode is formed on the planarsurface of the p-conductivity type region and another electrode, in acircular band shape, is also formed on the planar surface of then-conductivity type region near the circular edge of this surface. Theband gap is narrowest at the planar surface and becomes wider as thedistance from this surface increases. This distribution of band gapwidth is most easily obtained by using segregation during crystalgrowth. Owing to this distribution of band gap width, light generated atthe p-n junction formed between the p and n conductivity type regions ishardly absorbed in the crystal and is externally emitted with highefficiency.

In another type of device, a p-conductivity type layer is epitaxiallygrown on the surface of an n conductivity type III-V mixed compoundsemiconductor substrate which has a narrower band gap than the surfaceof opposite side. A circular portion including a part of this layer,which is slightly thicker than the p conductivity type layer, is removedby mesa-etching so that a p-n junction is delimited by thismesa-etching. The crystal is shaped in a dome or hemisphere, and twoelectrodes are formed on the n- and p-conductivity type layers asdescribed for the type where the surface on which electrodes areprovided is planar, and the p conductivity type region is formed bydiffusion.

The first type of light emitting device, where the p-conductivity typeregion formed by selectively diffusing impurities is used directly asone of the two regions forming a p-n junction, has the disadvantage thatcarrier mobility is small and light emission efficiency is low due tothe coexistenance of donars, such as Te, and acceptors, such as Zn, inthe p-conductivity type region.

The second type of light emitting device, where the p-n junction isformed by epitaxial growth and delimited by mesa-etching, has thedrawback that the diode must be mounted face-down on an auxiliarymounting device having two surfaces, the difference in height of whichis equal to the height of the mesa, in order to connect the positive andnegative electrodes to their respective external leads.

This complicates the process and lowers the yield rate of fabrication.Furthermore, this type of mounting increases both the thermal andelectrical resistances. For this type of light emitting device, theelectrical contact resistance at the positive and negative electrodesand the series resistance component of the p-conductivity type Ga₁ _(-x)Al_(x) As layer cannot be significantly reduced, because the majoritycarrier concentration in the p-conductivity type layer and that in the nconductivity type layer are preferably 1 - 2 × 10¹⁸ cm⁻ ³, taking theinjection efficiency and crystallographical structure into account.Since the external quantum efficiency of these high output power lightemitting devices is ordinarily from 5 to 15%, high thermal andelectrical resistances are serious drawbacks of this type of lightemitting device.

BRIEF DESCRIPTION OF THE INVENTION

An object of this invention is, therefore, to provide a high power limitemitting device, the feasibility of which is high and at the same timethe assembly of which is easy.

Another object of this invention is to provide a new method for makingsuch a high power light emitting device.

According to the invention, a p-conductivity type and then an nconductivity type layer are successively grown epitaxially on asubstrate made of a mixed compound semiconductor crystal having a widerband gap than the two above-mentioned layers. A surface portion of theepitaxially grown layers which is not covered by a mask deposited on then conductivity type layer, at a position where a p-n junction is to beformed, is doped with p-conductivity type impurities so that a smalln-conductivity type region is surrounded by a region converted intop-conductivity type. The other side of the crystal is formed into ahemispherical shape, so that the n-conductivity type region is locatedat the central portion of the hemisphere.

According to another more advantageous mode of realization of thisinvention, a ditch reaching the n conductivity type layer is formed byremoving a peripheral portion of the p-n junction by selective etchingso as to isolate the p-n junction by air.

The invention will be better understood from the following detaileddescription thereof taken in conjunction with the drawing. It should,however, be understood that the present invention is not limited to theparticular embodiments shown hereinbelow, but that various changes andmodifications can be made without departing from the spirit of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 5 are sectional views illustrating various stages in thefabrication of a high power limit emitting diode according to thisinvention.

FIG. 6 is a graph of the variation of the forbidden band gap width alongthe center line of the diode depicted in FIG. 5, and

FIG. 7 is a sectional view of another high power light emitting diodeaccording to the invention.

DETAILED DESCRIPTION Embodiment 1

FIGS. 1 to 5 illustrate various stages in the fabrication of a lightemitting diode made of Ga₁ _(-x) Al_(x) As according to this invention.A Ga₁ _(-x) Al_(x) As substrate 1 was prepared by liquid phase crystalgrowth method. The conductivity type of the substrate can be either p orn. The mixing ratio was greater than 0.2 at one surface and increasedtoward the opposite surface. A p conductivity type Ga₁ _(-x) Al_(x) As 0≦ x < 0.2 layer 2, 20 to 30μ thick, doped with Zn, and an n conductivitytype Ga₁ _(-x) Al_(x) As 0 ≦0 x < 0.2 layer 3, 2 to 3 μ thick, dopedwith Te, were successively grown epitaxially on the surface of thesubstrate 1, each having a narrower forbidden band gap than thesubstrate 1. The thickness of the p conductivity type layer ispreferably between 20 and 30μ. A layer thinner than 20μ may contain manycrystal defects or stress at the upper surface due to mismatching of thelattice constant, as will be discussed later, and for a layer thickerthan 30μ, it is difficult to obtain an appropriate mixing ratio at theupper surface because of the great segregation coefficient of AlAs withrespect to GaAs. The thickness of the n conductivity type layer 3 ispreferably between 2 and 3μ. The thinner the layer, the better the heatgenerated at the p-n junction is dissipated. The layer should be, atmost, 3μ thick. However, for a layer thinner than 2μ, the formation ofan ohmic contact on it will be difficult. The impurity concentration ofp- and n-conductivity type layers was 2 × 10¹⁸ cm⁻ ³ and 1 × 10¹⁸ cm⁻ ³respectively. After having well washed and dried the crystal thusobtained, an Al₂ O₃ layer about 1000 A thick and a phosphosilicate glasslayer about 2000 A thick were deposited by chemical vapor depositionmethod on the n-conductivity type layer 3. A part of this double oxidelayer was removed, leaving a circle having a diameter of 150μ byphotoetching method so as to form a mask 4. The crystal was placed in aquartz ampoule and Zn impurities were diffused in the surface at 700° Cfor 1.5 hours using a diffusion source of ZnAs₂. By this process a p⁺conductivity type layer 5 having a diffusion depth of about 5μ wasformed and, at the same time, a p-n junction 6 formed between thep-conductivity type layer 2 and the n-conductivity type layer 3 wasbounded in a desired shape, as shown in FIG. 2.

After having etched away the mask 4, a new layer of phosphosilicateglass 7 about 5000 A thick was deposited on the surface of the crystalby chemical vapor deposition. An opening for an electrode for then-conductivity type layer was provided in this phosphosilicate glasslayer 7 by photoetching. An ohmic contact 8 of AuGe-Ni-Au was depositedin this opening by evaporation, as illustrated in FIG. 3. After havingremoved this phosphosilicate glass layer 7 by etching, a newphosphosilicate glass layer 9 about 5000 A thick was deposited and acircular band shaped opening was formed near the edge of the surface ofthe crystal by photoetching. Through this opening an ohmic contact 10 ofan alloy of AuZn or AnSbZn was formed by evaporation, as shown in FIG.4. The crystal was formed in a hemispherical dome shape by mechanicaland chemical polishing so that the center of the hemisphere wasapproximately in accordance with the center of the planar surface, asshown in FIG. 5.

FIG. 6 shows the variation of the forbidden band gap width along thecenter line of the diode indicated in FIG. 5. The variation of theforbidden band gap width of each layer is easily obtained by using thedifference of the segregation coefficient of GaAs and that of AlAs,because the latter solidifies faster and has a wider forbidden band gapthan the former.

The light emitting dioe of this structure can overcome the drawbacks ofthe above-mentioned traditional techniques, namely, difficulty of thecontrol of wavelength of the emitted light and difficulty of assembly,simultaneously. The contact resistance of the ohmic contact formed onthe n-conductivity type layer can be reduced by increasing then-conductivity type impurity concentration at the surface. The contactresistance of the ohmic contact formed on the p⁺ conductivity type layeris very low, because the impurity concentration at the surface of thislayer formed during the process of isolation diffusion is very high. Inthis way, one can prevent the saturation phenomena in the output powerof generated light due to heat production, which is a characteristic ofhigh power light emitting diodes according to prior art techniques.

Furthermore, for light emitting diodes, it is more advantageous to forma p-conductivity type layer at first and then an n-conductivity typelayer as indicated above than to form a p-conductivity type layer on ann-conductivity type substrate in accordance with the prior arttechniques. The reason for this is believed to be as follows.

In general, the mobility of electrons is greater than that of holes insubstances used for injection type light emitting semiconductor diodes,such as GaAs,Ga₁ _(-x) Al_(x) As, and other mixed crystal compoundsemiconductors which have a direct transition type band structure.Consequently, the number of electrons injected into the p-conductivitytype region forming a p-n junction is greater than the number of holesinjected into the n-conductivity type region. Therefore, light isgenerated mostly in the p-conductivity type region. Thus, in order toincrease the light emission efficiency, it is very important to improvethe crystallographical properties of this region. For this purpose, itis advantageous that the p-conductivity type region be very thick sothat, near the upper surface of this region, there exist practically nocrystal defects due to the difference in lattice constants and so forth,which defects can exist in abundance near an interface between asubstrate and a layer epitaxially grown on it. Another reason consistsin the fact that carbon is an amphoteric impurity with respect to III-Vcompound semiconductors. It behaves as n-type impurity in a lowertemperature region and as p-type impurity in a higher temperatureregion. III-V compound semiconductor crystals are often grown in a jigmade of graphite and some carbon is inevitably introduced into thesecrystals during the crystal growth process. In the structure describedabove, since the p-conductivity type region is placed inside then-conductivity type region, the temperature is higher in the former thanin the latter. Consequently, carbon impurities are not a problem, evenif some carbon is contained in the light emitting region.

Several characteristics of diodes thus assembled were measured and itwas verified that the light output was about 50 mW (for a direct currentof 300 mA), the peak wavelength of the emitted light was 8020A, thethermal resistance was about 20-25 deg/W, and the saturation phenomenaof light output were not so evident as for traditional type devices dueto the fact that the thermal resistance was reduced. The yield rate forthe assembly was also increased to 95%, which was greater by about 25%than the yield rate for traditional devices which is 70%. The I-IVcharacteristics, which are the most important electric properties, wereas good as those of traditional type devices.

Embodiment 2

In Embodiment 1, Zn impurities were diffused into the crystal. In thisway, the interface between the n-conductivity type region 3 and thep^(+-conductivity) type region 5 is curved not only due to normallateral diffusion but also due to extraordinary diffusion between adiffusion mask and a semiconductor body. This may sometimes shorten thelife of the diode by strengthening the electric field at the edge of thep-n junction formed between the n-conductivity type layer 3 and thep-conductivity type layer 2. This disadvantage is obviated by using ionimplantation techniques.

After having formed the mask 4, Zn⁺ ions were implanted in the crystal,the temperature of which was maintained between room temperature and400° C using the mask 4 with a dose between 2.5 × 10¹⁵ cm⁻ ² and 2 ×10¹⁶ cm⁻ ². The mask 4 intercepted the Zn ions and the surface region,which was not covered by the mask 4, was converted into ap^(+-conductivity) type region. The phosphosilicate glass layer on theAl₂ O₃ layer was removed by using an etching solution of HF : NH₄ F = 1: 6. After having washed and dried the crystal, an SiO₂ layer about 2000A thick was deposited by chemical vapor deposition on the Al₂ O₃ layerand on the exposed surface of the p^(+-conductivity) type layer. Thecrystal was then put in a vacuum sealed quartz ampoule and heated for aperiod of 150 minutes at a temperature of 700° C. By this process theimplanted Zn ions were diffused so as to form a p^(+-conductivity) typelayer 5 reaching the p-conductivity type layer 2, so that the remainingpart of the n-conductivity type layer 3 was completely surrounded by thep^(+-conductivity) type layer 5 and a well-defined p-n junction 6 wasformed, as indicated in FIG. 2. A part of the crystal was cleft in orderto examine, by using a scanning electron microscope, how the p-njunction was formed, and it was observed that there was no extraordinarydiffusion at the interface between the Al₂ O₃ layer and the compoundsemiconductor crystal and that the p^(+-conductivity) type layer formedby diffusion reached well into the p-conductivity type layer 2.

After the formation of the p^(+-conductivity) type layer the diode wasassembled just as in Embodiment 1. The high power light emitting diodethus obtained had mechanical and electrical properties identical to thatobtained in Embodiment 1 and a longer life.

Embodiment 3

The diode obtained in Embodiment 1 an be improved by mesa-etching forisolating the n-conductivity type layer 3 by air. A circular band-shapedopening was formed in the phosphosilicate layer 7 so as to expose anarrow region of the surface of the crystal, which comprised thecircular boundary between the n-conductivity type layer 3 and thep^(+-conductivity) type layer 5. The crystal was dipped in an etchingsolution (temperature 18° ± 0.2° C) having a composition of ethyleneglycol : hydrogen peroxide : sulfuric acid = 7 : 2 : 1 for a period ofabout 10 minutes under agitation. In this way a ditch 11 about 5μ deepwas obtained. Since the n conductivity type layer 3 was thinner than 3μ,it is evident that a part of the p-n junction was etched away. Anotherphosphosilicate layer 12 about 6000 A thick was deposited on the crystalin order to passivate the surface, which layer was removed except forthe region in the ditch by photoetching method. The crystal was formedinto a hemispherical dome by mechanical and chemical polishing, as shownin FIG. 7, and as described in Embodiment 1.

In the above Embodiments, liquid phase epitaxial growth of Ga₁ _(-x)Al_(x) As on a Ga₁ _(-x) Al_(x) As substrate is described. However, itis evident that the method described above can be applied to othercombinations such as GaP-Ga₁ _(-x) In_(x) P (0.3 < x < 1), GaP-GaAs₁_(-x) P_(x) (0 < x < 0.4), etc.

We claim:
 1. A high power light emitting semiconductor devicecomprising:a semiconductor body which has first and second surfaces andwhich containsa first semiconductor region having a third surface, theforbidden band gap width of said first semiconductor region increasingfrom said third surface into said first region, a second semiconductorregion of a first conductivity type, disposed on said third surface ofsaid first semiconductor region, and having a forbidden band gap widthwhich is narrower than the width of the forbidden gap of said firstregion at said third surface thereof, a third semiconductor region of asecond conductivity type, opposite said first conductivity type,disposed on a prescribed surface portion of said second semiconductorregion and defining a first PN junction with said prescribed surfaceportion of said second semiconductor region, the width of the forbiddenband gap of said third semiconductor region being narrower than thewidth of the forbidden gap of said first semiconductor region at saidthird surface, and a fourth semiconductor region, of said firstconductivity type and having an impurity concentration greater than thatof said second semiconductor region, disposed on a portion of thesurface of said second semiconductor region, which portion of thesurface of said second semiconductor region surrounds said prescribedsurface portion of said second semiconductor region, the width of theforbidden band gap of said fourth semiconductor region being narrowerthan the width of the forbidden gap of said first semiconductor regionat said third surface; a first electrode disposed on the surface of saidthird semiconductor region; and a ring-shaped second electrode disposedon the surface of said fourth semiconductor region so as to surroundsaid first electrode; and wherein the surfaces of said third and fourthsemiconductor regions upon which said first and second respectiveelectrodes are disposed form the first surface of said semiconductorbody, while the second surface of said semiconductor body, which extendsto the first surface thereof, is hemispherically shaped.
 2. A high powerlight emitting semiconductor device according to claim 1, wherein saidfirst conductivity type is p-type and said second conductivity type isn-type.
 3. A high power light emitting semiconductor device according toclaim 1, further comprising a ring-shaped ditch separating said thirdsemiconductor region from said fourth semiconductor region.
 4. A highpower light emitting semiconductor device according to claim 3, whereinsaid ditch extends partially into said second semiconductor region andis contiguous with both said prescribed surface portion thereof and thesurface portion of said second semiconductor region surrounding saidprescribed surface portion.
 5. A high power light emitting semiconductordevice according to claim 1, wherein said fourth semiconductor region iscontiguous with said third semiconductor region and defines a second PNjunction therewith.
 6. A high power light emitting semiconductor deviceaccording to claim 5, wherein the maximum distance from said firstsurface of said semiconductor body to the interface between said secondand fourth semiconductor regions is greater than the distance from saidfirst surface of said semiconductor body to said first PN junction.
 7. Ahigh power light emitting semiconductor device according to claim 4,wherein the distance from said first surface of said semiconductor bodyto the interface between said second and fourth semiconductor regions isgreater than the distance from said first surface of said semiconductorbody to said first PN junction.
 8. A high power light emittingsemiconductor device according to claim 2, wherein the thickness of saidsecond semiconductor region is 20-30 μ and the thickness of said thirdsemiconductor region is 2-3 μ.